fix: restore local volume on unmute

[c436zhan]

Summary

Restore the local player's previous non-zero volume when the speaker button unmutes from zero, instead of jumping to the hardcoded 0.8 value.

Root cause

The local volume button used setVolume(volume > 0 ? 0 : 0.8), so any unmute from zero ignored the user's previous slider position.

Changes

• Track the last non-zero local volume while PlayerBar is mounted.
• Fall back to 0.5 when unmuting with no previous non-zero value.
• Add regression coverage for both previous-volume restore and no-history fallback.

Testing

• corepack pnpm --filter frontend test -- PlayerBar.test.tsx (fails before the fix with 0.8, passes after: 22 tests / 123 frontend collected tests passed)
• corepack pnpm --filter frontend typecheck (pass)
• corepack pnpm --filter frontend lint:boundary (pass)
• corepack pnpm --filter frontend lint (pass)
• corepack pnpm --filter frontend test (pass: 18 files / 123 tests)
• git diff --check (pass)

Note: the root pnpm verify script cannot run directly in this environment because nested scripts call a non-executable pnpm shim, so I ran the equivalent commands through corepack pnpm. The unchanged hub unit suite was also checked; test/admin-routes.test.ts consistently times out in admin — sync > POST /api/admin/hub/sync → returns 200 with local + peers result shape, unrelated to this frontend-only volume change.

Fixes #207

[benders]

FROM @claude:

Thank you for this fix, @c436zhan — the root-cause diagnosis was spot on, and your component-level tests that drive the real Mute/Unmute buttons were the nicest test approach of the two competing PRs; we ported that style into the merged branch (4b5c706) with credit to this PR.

Closing as a duplicate of #258, which was merged for issue #207. The deciding differences: the volume slider is square-law (position = sqrt(volume)), so the 0.5 raw fallback here renders at ~71% of the slider rather than halfway (the merged fix uses raw 0.25 = 50% position); #258 also extends remember/restore to the Sonos cast speaker button, and tracking every slider change in a ref meant a drag-to-zero left a near-silent last sample (e.g. 0.02) as the restore level.